1. FIELD OF THE INVENTION
The present invention relates to a drive circuit for a speed adjustable fan, particular to a drive circuit, which is disposed in a fan to control the current passing through coils of the fan so as to adjust the speed of the fan without any other outside circuit.
2. DESCRIPTION OF RELATED ART
As shown in FIG. 1, a prior art of drive circuit for a fan comprises a 4-pin Hall sensor (IC1'), a computing IC (IC2'). It is noted that the designation of IC in the specification is an abbreviation of integrated circuit. The power source connects a third pin of the computing IC (IC2') and connects a first pin of the Hall sensor (IC1') through a resistor (R1') respectively. The second and fourth pins of the Hall sensor (IC') are connected to the fourth and fifth pins of the computing IC (IC2') respectively. The sixth pin of the computing IC (IC2') is grounded through a capacitor (C). The seventh and ninth pins of the computing IC (IC2') connect two coils (V1', V2') respectively to drive the fan. The other ends of the coils (V1', V2') are connected to the power supply through a protective diode (D'). However, once the coils and the power supply in the circuit have been fixedly connected to each other, the speed of rotation can not be changed to suit the requirements of user's system. It is a trend that the speed of a fan can be changed in accordance with the user's command. It is known that heat dissipation by way of a fan is not limited under a condition of maximum load. Moreover, the problem of noise by using the fan will generate as well even under a condition of minimum load. Therefore, to improve the prior art circuit becomes necessary in order to meet the regulations of energy and noise for the environmental protection.
In view of the drawbacks of the prior art drive circuit for a fan, the inventor has dedicated in research for improvement. After having conducting various designs, investigations and prototype productions and revision based on professional experiences of the inventor, the drive circuit for a speed adjustable fan according the present invention has been developed advantageously.